Global positioning system (GPS) has been used for navigation and survey purposes for civilian and military applications. Global positioning system uses a constellation of orbiting satellites or space vehicles (SVs) that send radio signals to earth. The current constellation includes at least 24 operational satellites, which provides between five to eight space vehicles visible from any point on earth. Using the signals received from at least four of the visible space vehicles (the ensemble), an earth position (X,Y,Z) and time of a user can be computed. Typical applications of a global positioning system includes navigation aide and surveying.
There are two global positioning system services available. The Precise Positioning Service (PPS) is intended for military and select government agency users. The accuracy of the Standard Positioning Service (SPS), designated for civilian applications, is intentionally degraded to provide predictable accuracies of only 100 meters in the horizontal plane. The mechanism by which this intentional degradation is achieved is called selective availability (SA). Selective availability has two components: satellite clock dither (.delta.) and manipulation of the broadcast ephemeris data (.epsilon.). The selective availability bias on each satellite signal is different, resulting in a position solution that is a function of the combined selective availability bias from each space vehicle in the ensemble. Selective availability constitutes the largest component of error in the position solution.
Differential global positioning systems have been used to correct selective availability errors to improve the accuracy of the position solution. The differential technique uses bias errors measured by a receiver at a known position to correct the bias errors at a receiver with an unknown position. Therefore, differential global positioning system removes correlated errors measured by two or more receivers from the same ensemble of space vehicles. This differential technique is known as the local area differential global positioning system (LADGPS). However, local area differential global positioning system is useless when the user is remotely located from reference receivers.
When accuracy is required for users over a large region, a second differential technique known as the wide area differential global positioning system (WADGPS) may be used. This technique uses a network of intercommunicating reference stations that work in conjunction to overcome errors originating from sources such as selective availability. However, the control and communication schemes for the network of reference stations are complex and costly to operate.